A new technique for determining the presence of steeply dipping fractures and measuring vertical length and azimuth of zones of fractures intersected in deep wells is based on measurement of the azimuth of hole cross-section elongation at zones of overbreak where the drill has encountered a fracture. These measurements are recorded on the caliper part of a Schlumberger 4-arm dipmeter log. Two factors indicate that the sporadic overbreak zones are fracture controlled: (1) parallelism between azimuths of hole cross-section elongation at points of overbreak and directions of jointing measured at outcrops near the wells logged, and (2) influence of hole geometry at the zones of overbreak on logging-tool rotation. Downhole photographic verification of fracture control of direc ional overbreak has not yet been obtained.

The parallelism between joint sets at outcrops and azimuths of hole elongation at zones of overbreak has been demonstrated on the structurally simple Alberta plains where two orthogonal systems of extension joints are present. The dominant joint system (system I) has sets striking northeast and northwest, and system II has sets striking north-south and east-west. Of a total of 246 caved zones from 23 wells, scattered throughout Alberta, over 90% show preferential breakage in a northwest-southeast direction, parallel with one of the principal joint sets. It is believed that this technique for measuring subsurface fractures is applicable to other areas.